CS5323 datasheet(14/16 Pages) ONSEMI

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CS5323

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Then choose the inductor value and inherent resistance

to satisfy L/RL = R × C.

For ideal current sense compensation the ratio of L and

RL is fixed, so the values of L and RL will be a

compromise typically with the maximum value RL

limited by conduction losses or inductor temperature

rise and the minimum value of L limited by ripple

current.

3. For resistive current sensing choose L and RS to

provide a steady state ramp greater than 25 mV.

L RS + (VIN * VOUT)

TON 25 mV

Again the ratio of L and RL is fixed and the values of

L and RS will be a compromise.

4. Calculate the high frequency output impedance

(ConverterZ) of the converter during transients. This

is the impedance of the Output filter ESR in parallel

with the power stage output impedance (PwrstgZ)

and will indicate how far from the original level

(ΔVR) the output voltage will typically recover to

within one switching cycle. For a good transient

response ΔVR should be less than the peak output

voltage overshoot or undershoot.

DVR + ConverterZ

ESR

ConverterZ +

PwrstgZ

ESR

PwrstgZ ) ESR

where:

PwrstgZ + RS

CSA Gain 3

Multiply the converterZ by the output current step size

to calculate where the output voltage should recover to

within the first switching cycle after a transient. If the

ConverterZ is higher than the value required to recover

to where the adaptive positioning is set the remainder

of the recovery will be controlled by the error amp

compensation and will typically recover in 10 − 20 μs.

DVR + DIOUT

ConverterZ

Make sure that ΔVR is less than the expected peak

transient for a good transient response.

5. Adjust L and RL or RS as required to meet the best

combination of transient response, steady state output

voltage ripple and pulse width jitter.

Current Limit

When the sum of the Current Sense amplifiers (VITOTAL)

exceeds the voltage on the ILIM pin the part will enter hiccup

mode. For inductive sensing the ILIM pin voltage should be

set based on the inductor resistance (or current sense

resistor) at max temperature and max current. To set the level

of the ILIM pin:

6. VI(LIM) + R IOUT(LIM) CS to ILIM Gain

where:

R is RL or RS;

IOUT(LIM) is the current limit threshold.

For the overcurrent to work properly the inductor time

constant (L/R) should be ≤ the Current sense RC. If the

RC is too fast, during step loads the current waveform

will appear larger than it is (typically for a few hundred

μs) and may trip the current limit at a level lower than

the DC limit.

Adaptive Positioning

7. To set the amount of voltage positioning below the

DAC setting at no load connect a resistor (RV(FB))

between the output voltage and the VFB pin. Choose

RV(FB) as;

RV(FB) + NL Position VFB Bias Current

See Figure 4 for VFB Bias Current.

8. To set the difference in output voltage between no load

and full load, connect a resistor (RV(DRP)) between the

VDRP and VFB pins. RV(DRP) can be calculated in two

steps. First calculate the difference between the VDRP

and VFB pin at full load. (The VFB voltage should be

the same as the DAC voltage during closed loop

operation.) Then choose the RV(DRP) to source enough

current across RV(FB) for the desired change in output

voltage.

DVV(DRP) + IOUTFL

CS to VDRP Gain

where:

R = RL or RS for one phase;

IOUTFL is the full load output current.

RV(DRP) + DVDRP

RV(FB) DVOUT

Calculate Input Filter Capacitor Current Ripple

The procedure below assumes that phases do not overlap

and output inductor ripple current (P−P) is less than the

average output current of one phase.

9. Calculate Input Current

IIN +

VOUT

IOUT

(Efficiency

VIN)

10. Calculate Duty Cycle (per phase).

Duty Cycle +

VOUT

(Efficiency

VIN)

11. Calculate Apparent Duty Cycle.

Apparent Duty Cycle + Duty Cycle

#of Phases

12. Calculate Input Filter Capacitor Ripple Current. Use

the chart in Figure 13 to calculate the normalized

ripple current (KRMS) based on the reciprocal of

Apparent Duty Cycle. Then multiply the input current

by KRMS to obtain the Input Filter Capacitor Ripple

Current.

Ripple (RMS) + IIN

KRMS

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